Analyzing the benefits of the A-R/hard coat combination
by John Young
Which is better the inside skeletal structure of a mammal (i.e., me) or the outside structure of an invertebrate (i.e., a lobster)? In the end, the environment the species lives in dictates which is better for them. What about the use of a hard coat to protect an anti-reflective (A-R) coating from abrasion? Should the hard coat be outside or inside the A-R layer?
When A-R coatings first hit the market 25 years ago, there was little thought of including a hard coat underneath the A-R coating. That was understandable then, however, because hard coatings were also in their infancy.
At the time it was thought that applying a hard coat over the top of an A-R coating would ruin the ability of the A-R coating to accomplish its intended task. This, of course, is due to the fact a hard coating has thickness and has an index of refraction, all of which interferes with the functionality of the carefully designed layers of quartz, metal chlorides and fluorides used in the A-R coating. Applying the hard coat onto the surface of the lens first doesn’t seem to make much sense either. After all, how can it protect the A-R coating from underneath it?
To better understand the concept, imagine an experiment using a pillow as the lens, a piece of tissue as an A-R coating and a piece of cardboard as a hard coat (see illustration). If we cover a pillow (the lens) with a piece of tissue paper (A-R), then push a finger into the pillow, it will go through easily. Think of this as a large scratch in an A-R coated lens. If, however, we slide a piece of cardboard behind the tissue and push against it, our finger won’t go into the pillow and thus not leave a very big hole in the tissue. In fact the hole, or the scratch, would be difficult to see.
It is this same concept that has allowed the creation of more abrasion resistant A-R coatings. The coating can still suffer damage, of course, but, in time, all lenses will see some damage no matter how careful the wearer is with their spectacles. Still, improving the abrasion resistance in this manner has proven very successful though not without drawbacks.
There is what is known as a “marriage” between the hard coating and the A-R coating. This refers to the many problems that we’ve all seen with some A-R coatings delaminating or crazing shortly after the prescription is dispensed. Properly preparing the surface of the hard coating for A-R and using a quality A-R and A-R application process are deep at the heart of this issue. Based on tests carried out here at COLTS over the past year, there are some systems available today offering durable A-R coatings.
Our tests have also shown that this combination of coatings on a lens can weaken the lens more than if only one coating was applied. In fact, there are some lenses carrying manufacturer warnings against this double treatment for fear the resulting lens would not meet FDA “drop-ball” impact requirements. This is an issue, again based on our testing.
Still, according to industry estimates, the A-R coated/hard-coated lens represents more than 90 percent of the A-R coated lenses sold today. It is without question the best direction, for the moment, for the optical industry. What’s next, though, is an interesting question. In the meantime, be vigilant and ask appropriate questions regarding any of the products you use.
John Young is an ophthalmic lens expert with more than 25 years experience in the optical industry. He has worked for several lens manufacturers, including American Optical and Essilor, and is the former technical director of the Optical Industry Association. His company, COLTS Laboratories, is a Clearwater, Fla.-based independent lens testing facility designed to provide thorough and accurate quality and performance evaluations of spectacle lens products. His clients include lens manufacturers, wholesale labs, independent research organizations, large retailers and independent dispensers. The lab was the first U.S. ophthalmic testing laboratory accredited by the American Association for Laboratory Accreditation. It is also a Safety Equipment Institute-accredited eye protection/safety test lab. Young can be reached by phone at (727) 725-2323 and by email at firstname.lastname@example.org.